Searching for oil or more generally hydrocarbons is becoming more demanding in terms of hardware and devices in recent years because oil and gas fields or reservoirs are located deeper or in places difficult to reach. Prospecting and exploiting hydrocarbon fields laying under deep sea has become customary and necessitates hardware which is more resistant to environmental challenges like fatigue and corrosion, which were less important previously.
For extracting oil or gas from fields lying below deep sea off-shore platforms are used which are anchored to the sea bottom and strings are used which are conventionally called risers.
These strings are immerged in the sea and are subject to movements caused by sea streams and surface wave movements. Because of these continuous and periodic movements of the sea the strings do not remain immobile, but are subject to lateral movements of small magnitude which can produce deformations in certain parts of the joint and must withstand loads which induce fatigue stresses in the tubes, with particular respect in the zone of the threaded joint. These stresses tend to cause ruptures in the tubes in the vicinity of the thread and there is a need to improve the fatigue resistance of the threaded joints.
At present, fatigue performance and design for threaded connections for the oil and gas industry are adapted and extrapolated from other engineering fields. There are no specific standards or design/dimensioning specifications yet. Basic concepts can be found in the British Standard/Code of practice BS7608 for Fatigue design and assessment of steel structures, and DNV Class B S-N curve. Solutions have already been proposed in the state of the art to increase fatigue life of the threaded joints.
Document EP1726861 discloses a method for manufacturing a threaded joint for an oil well pipe where micro-shot peening is applied to improve its fatigue fracture strength.
The described method increases fatigue life to achieve the objective by applying shot peening to the threaded joint under specific conditions:
Spraying pressure: 0.4 MPa. Spraying distance: 100 to 150 mm. Spraying time: about 1 sec/cm2. Shot particle material: carbon steel of 0.8 to 1.0% C (HRC60 or more). Particle diameter is selected as shown in Table 4 of this document. It proposes also a combination of carburizing and nitriding treatments which can further improve the fatigue fracture strength.
Still there is room for improvement of threaded joint fatigue life.